1. Field of the Invention
This invention relates to a cutting tool quality sintered article containing a high-pressure phase boron nitride second only to diamond in hardness and a method for the production of this sintered article.
The sintered article according with the present invention is suitable as a material for tools used for cutting quenched steel materials and other materials that resist cutting.
High-pressure phase boron nitride exists in two types: single crystal cubic boron nitride (hereinafter referred to as "cBN") which is synthesized under an ultra high static pressure above 50 Kb and temperature of 1,200.degree. C. with use of a catalyst and polycrystalline wurtzite type boron nitride (hereinafter referred to as "wBN") which is synthesized under an ultra high static pressure over 100 Kb without use of a catalyst or under an ultra high shock pressure produced by the explosion of a powerful explosive. Both types of high-pressure phase boron nitride possess hardness second only to that of diamond and are useful as raw materials for the production a sintered article for use in a cutting tool capable of grinding, shaving, and cutting metals, particularly iron type metals.
Diamond sintered tools possess high hardness and are outstanding as cutting tools. However, they have the disadvantage that they reacts with iron type metals at elevated temperatures. They are therefore not suitable for cutting iron type materials.
2. Description of the Prior Art
At present, cermet, ceramics, cemented carbide, wBN, and cBN or cBN-wBN sintered articles are used for cutting iron type materials. For high-speed and high-accuracy machining of such hard-to-cut materials as high-hardness quenched materials and Hastelloy materials, the cBN sintered article or the cBN-wBN sintered article, a substance capable of compensating for the drawback of cBN, proves to be particularly suitable.
This invention is directed to a sintered article having cBN-wBN as a main component thereof.
Concerning sintered articles having cBN and wBN as main components, various techniques have been published as follows.
Japanese Patent Publication SHO 52(1977)-19208 discloses a sintered article having grown cBN dispersed in a matrix of wBN and states to the effect that the particles of wBN most desirably measure 0.5 to 10 .mu.m in diameter. Japanese Patent Publication SHO 60(1985)-6306 discloses a sintered article formed of a solid solution comprising a high-pressure phase boron nitride of wBN or cBN transformed from wBN during the process of sintering and M[C, O], M[N, O], and M[C, N, O] and states to the effect that in the sintered article, the wBN accounts for not less than 10% by volume and has a particle size of not more than 10 .mu.m. In the foregoing statement, the symbol M stands for a metal of Group 4a or 5a in the Periodic Table of Elements. Japanese Patent Public Disclosure SHO 55(1980)-97448 discloses a sintered article formed of a mixture of cBN and wBN and containing not less than 10% by volume of wBN. Japanese Patent Public Disclosure SHO 56(1981)-77359 discloses a sintered article formed of a mixture consisting of wBN and cBN wherein the wBN particles are 1 to 1.5 .mu.m in diameter and are contained in the high-pressure phase boron nitride in a concentration of 96-84% by volume. Japanese Patent Public Disclosure SHO 55(1980)-161046 discloses a sintered article produced by sintering a mixture of 1 to 40% by volume of wBN, ceramic, and a metal and consequently enables formation of a reticular structure consisting of cBN transformed from wBN, untransformed wBN, and the metal.
Japanese Patent Public Disclosure SHO 59(1984)-64737 discloses a sintered article of a mixture consisting of 60 to 95% by volume of cBN and 40 to 5% by volume of wBN wherein the cBN has an average particle diameter not less than 5 times the average particle diameter of the wBN. Japanese Patent Public Disclosure HEI 1(1989)-11939 discloses a sintered article containing 30 to 80% by volume of a high-pressure phase boron nitride which consists of 60 to 95% by volume of cBN having an average particle diameter of not more than 15 .mu.m and 5 to 40% by volume of wBN having an average particle diameter of not more than 5 .mu.m.
However, these sintered articles have the following problems.
The sintered articles disclosed in Japanese Patent Publications SHO 52(1977)-19208 and SHO 60(1985)-6306 and Japanese Patent Public Disclosures SHO 55(1980)-97448 and SHO 55(1980)-161046 all contain cBN which has been transformed from wBN. cBN of this nature causes problems regarding cutting tool performance. Moreover, the relation between the particle diameters of the cBN and the wBN, which bears heavily on the performance of cutting tool, is disclosed nowhere in these patent publications.
The sintered article disclosed in Japanese Patent Publication SHO 56(1981)-77359 is deficient in resistance to chipping and poor in cutting property because the wBN content of the high-pressure phase boron nitride is large. The sintered article disclosed in Japanese Patent Public Disclosure SHO 59(1984)-64737 manifests unreliable strength when used in a cutting tool because the cBN particles are unduly large and the wBN content is unduly high.
The sintered article disclosed in Japanese Patent Public Disclosure HEI 1(1989)-11939 has a problem regarding surface roughness because the cBN particles have a large average particle diameter in the range of 5 to 15 .mu.m and also a problem regarding resistance to chipping because the wBN particles have a relatively small diameter not exceeding 5 .mu.m and account for an unduly large proportion to the high-pressure phase boron nitride.
The sintered article of this invention consists of cBN, wBN, and a binding phase. Generally, cBN and wBN individually possess the following characteristics.
As described in the glossary of papers presented at the 1987 autumn general meeting of the Precision Engineering Society [Shinzo Enomoto, Masanori Kato, and Shinichi Miyazawa, "Cutting of iron type metal with cBN cutting tool (particularly the effects of particle diameter and content of cBN)," pages 649 to 650], cBN particles gain in binding force with the binding material in the sintered article and become less apt to separate from the sintered article so as to enable fabrication of a cutting tool with long service life when they have a large particle size and the cutting tool using cBN particles produces the most desirable finished surface roughness when the cBN particles have a very fine size.
The cBN particles have high cutting property because they possess sharp corners. They do not cleave but tend to chip because they comprise single crystals. The roughness of the finished surface is degraded in proportion as the diameter of the cBN particles used for the cutting tool is increased. wBN is a powder which is formed by the aggregation of primary particles, i.e. minute crystals some tens of nm in diameter. It, therefore, manifests a low cutting property, lacks cleaving property, and enjoys high toughness. Since the wBN particles have an extremely small size, they have the advantage of forming on a given work blank a finished surface of highly satisfactory roughness.